Amplifying system



y 1941- A. PREISMAN 2,243,121

AMPLIFYING SYSTEM Filed April 1, 1938 v 2 Sheets-Sheet 2 INVENTOR.ALBERT PRE/S MAN A T'TORNEY.

atented May 27, 1941 AMPLIFYIN G SYSTEM Albert Preisman, Bronx, N. Y.,

Corporation of Americ ware assignor to Radio a, a corporation of Dela-Application April 1, 1938, Serial No. 199,337

9 Claims.

This invention relates to amplifying systems, and more particularly, toamplifying systems having substantially uniform amplification over anexceedingly wide frequency band, together with zero phase delay shift.Such amplifiers are free from phase distortion as a function offrequency throughout the response range.

Such amplifiers are of considerable importance in television where it isnecessary to transmit all frequencies from zero up to megacycles ormore. In accordance with the invention, amplifying systems having aplurality of stages have their plate circuit constants so chosen withrespect to the grid circuit constants of the following stage, thatperfect freedom from phase distortion is obtained. It is well recognizedthat in wide band amplifiers such as those required for use intelevision, in contra-distinction to amplifying systems for sound, thephase relation between input and output of the various frequencycomponents present in the video signals must be rigorously maintainedbecause any relative phase displacement between these components resultsin a loss of definition in the reproduced picture. In this connection,it is exceedingly important that the low frequency components have theirinitial phase relation with respect to the higher frequency componentsfree from any variation or deviation, since the low frequency componentsrepresent the background values, while the high frequency componentsdetermine the sharpness or resolution of the reproduced image. Any phasedisplacement between these two sets of components destroys the sharpnessof detail, inasmuch as the high frequency components do not coincide inproper spatial relation with the background so that instead of a sharpand clear image, a fuzzy image is reproduced. By properly choosing theconstants of the plate circuit and the grid circuit of the followingstage in accordance with the invention, it is possible to provide anamplifying system which is completely free from such phase distortionand at the same time toimprove and increase the overall gain of theamplifier, even when conventional triode thermionic amplifying tubes areused.

It is further well known that the upper frequency limit of suchamplifiers is generally determined by the inter-electrode capacities ofthe tubes, and in order to avoid amplitude distortion, relatively lowplate impedances must be used so that the shunting capacities may notseriously affect the amplification adversely. Under such conditions, inorder to obtain sufficient gain, the

prior art has made frequent attempts to use pentode thermionicamplifiers. By the use of the present invention, amplifiers using triodethermionic tubes are capable of giving the increased gain over that ofthe pentode and at which was not possible in the prior art. Undercertain conditions, especially where tubes of very hightrans-conductance are used, it is necessary to provide low decouplingresistances so that the potential drop across the resistance will not beexcessive and require, in turn, excessively high plate supply voltages.Where this was done in the prior art, violent transients arose inoperation, due to imperfect correction of the phase of the amplifiedcurrents. With the present invention, however, this is completelyavoided so that high trans-conductance thermionic amplifiers can be usedwith conventional plate supply voltages, and yet provide substantiallyperfect freedom from any phase distortion.

The invention further provides the means for feeding an amplifier from atransmission line, such as a coaxial cable, without suffering any phasedistortion from this connection due to reflections and improperterminations by inserting between the transmission line and gridcircuit, a network representing an equivalent plate circuit network.

' These features of the invention herein to be described in detail, areobtained by taking into account the filter circuit of the power supplyfeeding the plate circuit of a thermionic tube which the prior artdisregarded, as well as the plate load impedance, and by selecting thegrid coupling and grid circuit network of the following thermionicamplifier so that it is similar in its impedance charac-terstic to thecombined plate load-plate filter circuit. By making the 7 two networkssimilar in their impedance characteristics, the phase angle of theenergy components fed between the grid and cathode of the secondthermionic amplifier is identical with the phase angle of the energycomponents impressed across the input of the first thermionic tube.Accordingly, the amplifier cannot introduce any phase distortion betweenthe various energy components, while at the same time it providesuniform amplitude of amplified. energy.

It is thus the main object of this invention to provide a new andimproved amplifying system which has substantially uniform amplitude andphase characteristics throughout the entire wide band of frequenciesnecessary to transmit high detail television images, although the samegeneral form system is applicable because of its uniformity and flatcharacteristic to all types of amplifiers.

Another object of this invention is to provide an amplifying systemhaving a plurality of stages in which the plate load-plate filtercircuit of one thermionic tube is made to have similar impedancecharacteristics to the grid couplinggrid circuit network of a followingthermionic amplifier.

Still another object of this invention is to provide an amplifyingsystem in which perfect phase compensation can be obtained even with lowdecoupling plate filter resistances.

Another object of this invention is to provide an amplifying system inwhich the frequency characteristic of the amplifier is independent ofthe grid circuit impedance of the thermionic amplifier comprised withinthe amplifying system.

Other objects and improvements of my invention will be ascertained upona reading of the following description taken together with the drawings.

In the drawings, Fig. 1 Shows schematically a circuit comprising atwo-stage amplifier embodying the circuit arrangement of the invention;

Fig. 2 shows schematically a modification of the embodiment shown inFig. 1, in which the D. C. component is not'transmitted and where thegrid circuit includes a grid bias filter;

Fig. 3 shows another embodiment of my invention wherein a transmissionline such as a coaxial cable, is used to feed the first stage of anamplifying system.

Figs. 4, 5 and 6 show diagrammatically equivalent circuits of that shownin Fig. 1 for purpose of explaining the invention and deriving thenecessary conditions and relationships between the circuit constants;

Figs. 7, 8, 9, l and 11 are schematicrepresentations of a furtherembodiment and modification of my invention useful where a twosectionplate filter is used, in contradistinction to the single section platefilter shown in Figs. 1 and 2.

Turning now to the drawings, the invention will be explained in detailas to the constituent parts and their functioning. In Fig. 1 there isshown a two-stage amplifier, although it is to be understood that morestages may be provided as well as less stages, such as, for example, thesingle stage amplifier shown in Fig. 3. In Fig. 1 the input signalcomprising a plurality of frequencies is impressed on the terminals I soas to actuate the grid 1, cathode 5, circuit of the tube 3. In the plate9 circuit is connected serially an inductance I3, commonly called apeaking coil, and a plate load resistance [1 to the filter of decouplingcircuit comprising the resistance l and condenser l3 connected to thebattery H which supplies the plate circuit. The second stage of theamplifier comprising the thermionic triode 3| is coupled from thepreceding tube 3 through a parallelly connected resistance 2| andcondenser 23, and the grid 35 of the tube 3| is connected to one side ofthe resistance 25, the other side of the resistance being connected to aparallelly connected resistance 21 and condenser 29, which in turn isconnected to a bias battery 39, said bias battery being connected tocathode 33. The load circult is connected appropriately to the terminals4 l In accordance with the invention, the elements l3, l5, l1 and I9having been chosen, the constants 2 I, 23, 25, 21 and 29 are determinedin accordance with the principles as will be outlined below so that thegrid network comprising the grid coupling resistance and condenser 21and 23, respectively, and the grid circuit comprising the elements 25,21 and 29, are similar in impedance characteristic to the plate circuit,differing therefrom only in magnitude by a real number. Under suchconditions, the energy components impressed between the grid 35 andcathode 33 of the tube 31 will have identical phase relation withrespect to each other as the energy components impressed between thegrid 1 and cathode 5 of the amplifier 3.

In order to show what this relationship must be, reference will be madeto Fig. 4 inwhich Z'o represents the dynamic plate impedance, Z1represents the plate load together with an equivalent portion of theplate supply filter, Z2 represents the difference between the values ofthe plate filter and the equivalent portion of the plate filter includedin Z1. mZz represents the grid coupling circuit and mZr is equivalent tothe grid resistance plus the parallelly connected condenser andresistance connected thereto between the grid and cathode. sents thevoltage impressed by the tube, while e1 represents the voltage appearingbetween the plate and cathode. e2 represents the voltage impressedbetween the grid and cathode of the following stage. It will readily beseen in accordance with application of elementary circuit analysis, suchas Kirchhoffs laws, that It will be noted from the above equation thatthe numerator is equal to the denominator, and

6o reprethese cancel out, except for the terms S and those including in.m is likewise a constant and real number and is chosen as will bepointed out below, as the ratio between the load resistance and thetotal grid resistance. Under such circumstances, it will be at onceapparent that 62, that is, the voltage impressed across the grid circuitof the second tube, is equal to the grid voltage impressed across the.grid circuit of the first tube multiplied only by a real number.Accordingly, the voltage impressed upon the grid of the sec ond tube isindependent of frequency and thus the amplification cannot introduce anyphase distortion. Thus it only remains to show the relationship whichmust be provided in order that the grid circuit elements shall besimilar to those of the plate circuit elements.

To facilitate this, reference is made to Fig. 5 in which forconvenience, the circuit shown in Fig. 1 has been simplified. In orderthat the plate circuit may be similar to the grid circuit, the branch ofthe plate load circuit must be broken up into two parts in such a mannerthat part of CF and RF can be associated with R1. to form Z1 and theremainder to form Z2. Then Cg and Egg will be made m times the latter,and Rg together with Rb and Cb will be made equal to m times Z1. It willbe noted that Z is equivalent to Tp and 60 is here #61, so that thebreaking up of C and F into component parts is such as to satisfyEquation 4.

In Fig. 6 there is shown the partition of the circuit Cr and Rf whichcorresponds to the elements l3 and IBshown in Fig. 1 in their componentparts. By so choosing it will be apparent that there is provided theequivalent of a Wheatstone bridge and that no current will flow alongthe path indicated by the dotted line from the junction point of the twocondensers and the junction point of the two resistances, since thebridge is balanced. Accordingly, these two points may beshort-circuited.

It will thus be apparent that a further equivalent bridge is provided inwhich one arm C2-R2 has the equivalent characteristics of the diagonalarm Rpg cg while the arm comprising R1. connected in series with theparallelly connected C1 and R1 is equivalent to the diagonal armcomprising Rg, Rb, and Cb. In this connection it is to be noted that theseries inductance or peaking coil is effective only at the very highestof frequencies, and so at the low frequencies, can be neglect-ed and,consequently, is not shown in Figs. 5 and 6. At the very highfrequencies at which the peaking coil becomes effective, the variouscapacities as C1, C2, Cb, and Cg have exceedingly low reactance andconsequently, the amplifying system acts as though it was purelyresistance coupled, and consequently, substantially no phase distortionis introduced even at these frequencies. It is well known in the priorart that such peaking coils together with their distributed capacity,are worked within the region at which substantially no phase shiftdistortion between the various frequency components of the energyamplified is introduced, so that the neglect of the taking into accountthe inductance in the plate circuit does not invalidate the results andconclusions above given.

' The determination of the various parameters is governed by thefollowing conditions.

(1) The high frequency response of the stage determines the value of RLand the peaking coil in series with it. Hence the ratio is determined bythe high frequency response. Experience has shown that this ratio is ofthe order of 1/5.

(2) The values of RF and CF are determined by feedback considerationsand must be large enough to prevent motor-boating in multi-stage units.Commonly used values are on the order of 10,000 ohms and 8 micro-faradsrespectively.

(3) The maximum value of m is determined by the value of R1. and thetotal grid resistance Rg and Rh. The total grid resistance comprising Hgand Rb in turn is determined by the amount of the so-called grid gascurrent, i. e. the small value of grid current which flows between gridand cathode, even when the tube is biased negatively. Generally thisresistance cannot exceed one megohrn in order to prevent the gridcurrent from affecting adversely the amplification characteristic of theamplifying system. Small values of m are permissible, however, and mayeven be desirable in certain cases.

(4) With the above values fixed by external considerations, We can nowdetermine the values of Rpg, Cg, Rg, Rb, and Cb. By algebraicmanipulation it can be shown that a F m+1+mk] An example will indicatethe range of values to be expected for the various parameters. Thus,assume Tp=10,000 ohms-a usual value for a triode. Suppose for properhigh frequency response, R1. must be 2,000 ohms. Then k=0.2. Assume thatnecessary values for CF and RF are 8 mfds. and 10,000 ohms respectively,and that m can be 100. Then, applying Equations 6, 7, 8, 9, V

100 1 R 19,000': 1 i g]=834,0{){) ohms R,,= 100 X2,000=200,000 ohms0.484 mfd.

A special case arises when k is approximately zero, which arises uponthe substitution of a pentode tube for a triode. In such case, theinternal impedance being so very high, makes likewise become zero, anddrop out of consideration and that Cg reduces to and R =m(RF) while Rgremains equal to mRL. Accordingly, if it is desired to use pentodes inplace of triodes, the circuit elements 21 and 29 shown in Fig. 1 can beeliminated.

Where it is not necessary to amplify the D. C. component, representingthe average light value, the circuit arrangement shown in Fig. 2 may beused, in which the elementsbearing the same number are identical tothose shown in Fig. 1. It will be noted in this case, that a blockingcondenser 43 is provided in series with the grid coupling resistance 21.The value of this capacity is made relatively large so that itsimpedance is negligible compared with the resistance of the resistor 21.In the example cited above, a condenser of 1' 6 of a micro-farad wouldhave negligible reactance compared to the calculated 834,000 ohmresistance down to 15 cycles. Sufficiently below this frequency,however, the effect would be attenuation of response, together withappreciable phase shift. At the same time, it is to be noted that inFig. 2 a power-pack bias supply is shown for tube 31. In this case, thebias determined by the position of the tap 19 on the voltage divider 11includes resistance 8| and this resistance is equivalent to theresistance 21 shown in Fig. 1. By appropriately choosing this value tocoincide with that determined above as Rb, the conditions necessary togive freedom from phase distortion is met.

A further advantage rises from the use of the circuit shown in Fig. 2where it is unnecessary to transmit frequencies below 15 cycles in thatthe characteristic indicates a fairly sharp cut-off below 15 cycles, inwhich region motor-boating is most apt to take place, so that thecircuit shown in Fig. 2 provides the additional advantage of effectivelysupplying more filtering and freedom from self-oscillation of theamplifier.

It is evident from the above that if freedom from phase distortion is tobe obtained in accordance with the invention, the grid of one tubecannot be used to compensate for the latters plate circuit, but that thegrid circuit of the following tube must be used for this purpose. If theinput circuit of the first stage of a multi-stage amplifier is to be fedfrom a source, as a coaxial cable, then for perfect compensation anartificial plate circuit network must be used to terminate the line fromwhich the grid is appropriately fed. Such a connection is shown in Fig.3, where the artificial plate circuit is shown as the elements 53 and55, together with the coupling circuit 51, 59. In such a case, thecharacteristic impedance of the coaxial cable 45 replaces, or can besubstituted for Z0 in the discussion above, or for Tp as shown in Fig.5.

If a two section decoupling circuit be employed, as shown in Figure '7,then the grid coupling circuit can be determined as follows:

The decoupling circuit can be transformed into an equivalent circuit.This results in a circuit shown in Figure 8. This circuit can then bebroken up as shown in Figure 9. Then ZA is made m times the impedance ofthe bottom portion, and Z3 is made m times R1. plus the top portion.Then q is determined so that Equation 4 is satisfied. The result is agrid circuit of the form shown in Figure 10. This may be transformedinto an equivalent form shown in Figure 11.

The values of the various parameters are as follows: For Figure 10:

m 1 mic 1) (12) R,,=mR,, (l4) nidil'l Q m +1 R. 2 (15) m(m+ 1) 1i R'...R.+ 2 16) m l mic C,,= C (l7) (Rain) m l (18) I ri z) R R, m+1+mk (19)C' =[fl i l lfl Q 20 m It lfi) For Figure 11:

m+ 1 mk C (21) m(m+ 1) R 1Ri[ (22) [m+1+mk (23) m(m+ 1) CI2 [m+ 1 +mlc](24) R0 MR1, m lc R"i= 1 (26) m k R [m+l+mlc (27) m 1 mk c '2 c.[,,,.,,

mic GI- (29) In all these formulas Ja -Rum.

At this point it may be well to point out that the grid coupling circuitof Figure 2 can also be transformed, if desired, into equivalent circuitsimilar to that shown in Figure 10.

I: mic

m Comparing Equation 30 with Equation 11, which gives the gain for thetriode tube, it will be noted that the denominator of Equation 30includes in its sum the addition term k, which factor is not present inthat of the denominator showing the gain for the triode, and hence,since 70 is a real positive number, the denominator of Equation 30 mustbe larger, with the result that the gain is smaller than that given byEquation 11.

It will be further noted that no limitation is placed on how low thegrid coupling impedance may be as far as frequency response isconcerned. The only effect of a low grid coupling impedance will be areduction in the gain. It is further to be noted that the circuitsdescribed above enable perfect compensation to be obtained, even if thedecoupling time constant is low, and this,- together with the use oftriode tubes and providing, at the same time, higher gains than can bere alized from pentodes, affords a superior amplifying system,particularly useful for wide band amplification.

Having described my invention, what I claim is:

1. In an amplifying system wherein is provided a thermionic tube havinga cathode, control electrode, and anode, the method of preventing phasedistortion in the system: which comprises providing a first complexbifurcated impedance path between the cathode and control electrode oneof the forks of said bifurcated path varying substantially withfrequency, providing energy to be amplified from a source of energyhaving a plurality of frequency components, and interposing between thesource of energy and the first complex impedance path a second. complexbifurcated impedance path whose impedance differs only in magnitude by areal number from the impedance of the first impedance path.

2. In an amplifying system wherein is provided two thermionic tubes eachhaving a cathode, grid, and anode, the method of avoiding phasedistortion which comprises providing a complex bifurcated impedance pathbetween the anode and cathode of one of said tubes, one of the forks ofsaid bifurcated path varying substantially with frequency, supplyingenergy to the complex impedance path through the named tube, supplying asimilar complex bifurcated impedance path whose impedance differs onlyin magnitude by a real number from the impedance of the first namedimpedance path, and energizing the grid and cathode of the other of thetwo tubes through the similar complex impedance path from the firstnamed complex impedance path.

3. An amplifying system for preventing phase distortion comprising athermionic tube having a cathode, control electrode, and anode, meansfor providing a first complex impedance path including parallellyconnected current conducting elements between the cathode and controlelectrode,

one of said elements varying substantially with frequency and another ofsaid elements being substantially independent of frequency, means forproviding energy to be amplified from a source of energy having aplurality of frequency cornponents, and means for interposing betweenthe source of energy and the first complex impedance path a secondcomplex impedance path whose impedance is similar to and differs only inmagnitude by a real number from the impedance of the first impedancepath.

4. An amplifying system for preventing phase distortion comprising twothermionic tubes each having a cathode, grid, and anode, means forproviding a complex impedance path including parallelly connectedcurrent conducting elements between the anode and cathode of one of saidtubes, one of said elements varying substantially with frequency andanother of said elements being substantially independent of frequency,means for supplying energy to the complex inrpedance path, means forsupplying a similar complex impedance path whose impedance differs onlyin magnitude by a real number from the impedance of the first namedcomplex impedance path, and means for energizing the grid and cathode ofthe other of the two tubes through the similar complex impedance path.

5. An amplifying system comprising a thermionic tube having an inputcircuit, a cathode, anode, and control electrode, an inductanceconnected in series with a resistance, decoupling resistance, andcondenser, between the anode and cathode, a second thermionic tubehaving a cathode, control electrode and anode, a parallelly connectedcapacity and resistance between the anode of the first named thermionictube and the control electrode of the second thermionic tube, and aserially connected resistance and decoupling resistance and capacityconnected between the grid and cathode of the second thermionic tubewhereby phase distortion in the amplifier is prevented.

6. An amplifying system comprising a transmission line, a thermionicamplifier having a cathode, control electrode and anode, a resistanceconnected in series with a decoupling resistance and capacity connectedbetween the grid and cathode of the thermionic tube, and a compleximpedance network having a shunt and series arm, said shunt armcomprising a resistance connected in series with a parallelly connectedresistance and capacity, and said series arm comprising a parallellyconnected resistance and capacity connected between the transmissionline and the control electrode and cathode of the thermionic tube.

7. In an amplifying system wherein is provided two thermionic tubes tobe connected in cascade and in which each thermionic tube has outputelectrodes and input electrode, the method of amplifying energysubstantially free from amplitude and phase distortion, which comprisesthe steps of providing a complex bifurcated impedance path between theoutput electrodes of one of said tubes, providing between the inputelectrodes of the other of plex bifurcated impedance path whoseimpedance differs only in magnitude by a real number from the impedanceof the first named complex impedance path, and supplying from the outputelectrodes of the first of said tubes energy to the input electrodes ofthe other of said tubes.

8. An amplifying system. comprising two thermionic tubes connected incascade and havsaid tubes a similar com-' ing output and inputelectrodes, means for providing a complex path including parallellyconnected current conducting elements between the output electrodes ofone of said tubes, one of said elements varying substantially withfrequency and another of said elements being substantially independentof frequency, means for providing between the input electrodes of theother of said tubes a similar complex impedance path whose impedancediffers only in magnitude by a real number from the impedance of thefirst named complex impedance path, and means for supplying from theoutput electrodes of the first of said tubes energy to the inputelectrodes of the other of said tubes.

9. An amplifying system comprising a thermionic tube having an inputcircuit, a cathode, anode, and control electrode, an inductanceconnected in series with a resistance, two decoupling resistances andcondensers, between the anode and cathode, a second thermionic tubehaving a cathode, control electrode and anode, a parallelly connectedcapacity and resistance between the anode of the first named thermionictube and control electrode of the second thermionic tube, and a seriallyconnected resistance and two decoupling resistances and capacitiesconnected between the grid and cathode of the second thermionic tubewhereby phase distortion in the amplifier is prevented.

ALBERT PREISMAN.

